SAN MARCOS  Growing up in the Twin Oaks Valley, Chris Johnson knew all the best tracks.

There was one by Jack’s Pond, two near the corner of Borden Road and Woodward Street, and several scattered around the hills of the modern-day Twin Oaks Golf Course. These were the dirt tracks of the 1990s where boys would show up with BMX bikes and shovels to shape the ground and test their jumping skills.

Bike jumping was Johnson’s reason to exist, in a sense — until the inevitable accident, a wreck that left him with a broken ankle and a bleak set of options.

Johnson, you see, was born with a rare condition called sacral agenesis, which led to deficiencies of nerves and muscle tissue in his lower legs.

Because of his disability, the doctors said restorative surgery would take a year to heal after his crash. One of them floated the word “amputation” — and explained that a prosthetic lower leg would be much more useful than his natural foot, anyway.

It wasn’t an easy decision for an 18-year-old, Johnson recalled: “When my foot was amputated, I’d get in my wheelchair and wheel down to the jumps. The jumps were my place — even though I didn’t have my bike and I couldn’t jump, I’d just go down there and hang out, watch all the other kids ride their bikes.”

Even after he healed, rid- ing never felt quite the same.

Until recently, that is.

Johnson was interning at Southern California Orthotics and Prosthetics last year when he absent-mindedly hopped across the lab on his prosthetic leg while having the brace on his other foot repaired.

“All of a sudden, a few of the practitioners broke out into a round of applause,” he recalled. “One of them asked if I had ever heard of the Challenged Athletes Foundation, and soon after that I applied.”

The foundation wanted him to train for six months to prove his commitment, which he did. So in addition to coaching and training, it gave him his first professional road bike six months ago. It was resting by his side when we met, having carried Johnson the five miles from his home to the Palomar College campus.

Few can say they learned to ride a bicycle in the same year that they learned to walk, but that is one of Johnson’s achievements. He was 6 — and the doctors had said he wouldn’t even be able to sit up as a baby.

I met him Thursday at Palomar before he headed off to an 11 a.m. session of his last junior college class before he transfers to Cal State San Marcos.

He told me he plans to earn his bachelor’s degree in psychology across the freeway, and then head north, to Cal State Dominguez Hills — home to a renowned prosthetics program.

“The experience I have had with the orthotics and prosthetics, it’s given me opportunities in life that I wouldn’t have had otherwise,” Johnson told me. “There’s nothing I want more than to give that back to other people.”

Besides, he added, “Practitioners who actually wear the prosthetics, they relate better. They do a better job.”

Johnson has entered two road races so far in San Diego County, and at 31 years old, he is training for his first century ride, a 105-mile route through Death Valley on March 2.

Of his initial competition last fall, he said, “When I got out there and actually started racing, I felt competitive. I was actually racing with able-bodied people and passing them. I knew I had what it took to ride a bike.

“Being a challenged athlete, going into a race you’re always apprehensive — how am I going to compete among able-bodied people? How am I going to rank among them?”

The Death Valley ride is limited to 300 riders, “and as far as I know, I’m the only challenged athlete who will be participating.”

To prepare, he is training five days a week at Coastal Motion Fitness in Oceanside and the college fitness center, besides riding 20 hours a week on the road bike provided for him by the Challenged Athletes Foundation.

When you think about it, Johnson’s story gives new meaning to the cliché that something — anything — could ever be “like riding a bicycle.”

To him, there is nothing like it.

“It’s such an exhilarating experience to get back on my bike and have that same feeling I did when I was a kid,” he said. “Every day, that’s what I looked forward to — I’d get out there and dig and work, and I’d build these awesome jumps. I’d go home that night thinking, ‘All right, the dirt’s going to be nice and dry tomorrow, and I’ll get on that jump.’

“I feel like I have that spark back now. When I’m getting ready to go out on my bike, I feel like that little kid again, getting ready to go jump.”

Know anyone with an interesting job, history or outlook on life? Contact Tom Pfingsten at fallbrooktown@gmail.com